1. Field of the Invention
The present invention relates to methods and apparatus for fabricating corrugated paperboard. More particularly, the present invention relates to a method and corresponding apparatus for applying back pressure to a single-faced corrugated web while in transit over a glue applicator roll. The applicator roll applies a film of adhesive to the exposed corrugation flute tips of the single-faced web in preparation for adding a second liner web to complete a double-faced assembly.
2. Description of the Prior Art
In the fabrication of corrugated paperboard, a first liner web or single facing is adhesively applied to the flute tips of the corrugated medium web while the medium web is still in intimate contact with the corrugating roll surface profile. Consequently, the single-facing web may be positively pressed into intimate contact with the corrugation flute crests of the medium by a high pressure roll nip without concern for either (a) crushing a portion of the corrugated web pattern by excessive contact pressure or (b) omitting a flute line of adhesion because of insufficient contact pressure.
After the single-facing web is applied and the corrugated medium stripped from the corrugated roll surface, the reverse face of the medium is no longer accessible to a corrugated backing profile in support of the medium for application of the double-facing web. Such absence of a positive backing support structure is particularly acute at the glue applicator station whereas adhesive for the double-facing web is applied to the exposed flute tips. Some pressure is required to hold the corrugated face of the continuously running, single-faced web assembly firmly against the applicator roll surface to assure that sufficient adhesive is applied along the full transverse length of each passing flute tip. However, excessive pressure will crush the fluted medium whereas insufficient pressure will permit skips in adhesive application.
Uncontrolled tendencies of a single-faced web assembly to warp transversely represents a major source of difficulty in back pressure regulation. When a fixed dimension gap setting is used between the glue roll surface and a corresponding backing roll surface to transversely flatten a warped, single-faced board for line contact with the flute tip glue applicator roll, a line of crushed flutes often results along either the board center or along either edge.
One prior art effort to control single-faced board back pressure at the double-face glue station has been to mount a backing roll approximately mid-span between two swing arms. One end of each swing arm is pivotally secured to the machine frame whereas the free ends of the arms are supported by fluid pressure. A fluid pressure control system supports substantially all of the roll weight applied against the singlefaced board.
More recently, it has been taught that a controlled air bearing pressure against a web line of single-faced board at the glue applicator roll provides a superior product in terms of reduced flute crushing and improved, full line, tip gluing. This teaching is represented by U.S. Pat. No. 4,617,076 to T. B. Jones, Jr. et al. Although the air bearing method of pressing single-faced board against a glue applicator roll is significantly superior to a rigid surface rider roll, room for further improvement remains. Working experience with such air bearing pressure source has revealed a tendency of the air bearing structure to bow away from the board plane from the middle to the ends. Flute crushing is eliminated but occasionally, the flute tip glue lines are incomplete along the board assembly edges.
Analysis of this air bearing malfunction suggested that the air bearing structure gets hotter along the bottom portion than the top. Asymmetric thermal expansion consequently produces a bow in the air bearing length to lift the bearing ends.
These air bearing assemblies have been constructed as an integral unit having wear shoes in intimate contact with an air supply manifold duct. A multiplicity of small diameter holes drilled through the shoe thickness and the duct floor vent supply air between the shoe base and the top surface of the running web of single-faced board. Theoretically, there should be no physical contact between the shoe base and the running board. In practice, however, conditions may develop to cause a light frictional drag of the board against the shoe base. Heat is thereby generated in the shoe structure which is conducted directly into the manifold floor. Such friction heat is generated more rapidly than the air flow can carry it away. Hence, the floor of the air bearing expands more than the top thereby causing the consequential bow.
In view of the aforedescribed operating difficulty with prior art air bearing designs, it is an objective of the present invention to provide an air bearing assembly that segregates the wear shoe portion of the unit from the air supply manifold portion.
Another object of the present invention is to longitudinally segment the wear shoe portion of an air bearing assembly into short, structurally independent units.